1. Field of the Invention
The present invention relates to a fire detector in which a light emission section is driven to glow at regular intervals, light emitted from the light emission section and varied depending on smoke stemming from a fire is received by a light reception section and sampled as a smoke detection signal, and a noise de-influence method for the fire detector.
2. Description of the Related Art
In photoelectric fire detectors for detecting smoke that stems from a fire, a light emission section is driven to glow at regular intervals. Light emitted from the light emission section and then scattered or attenuated due to smoke stemming from a fire is received by a light reception section, converted into an electric signal, and sampled as a smoke detection signal proportional to the concentration of the smoke.
The fire detectors fall into analog fire detectors and so-called on-off fire detectors. The analog fire detectors sample the smoke detection signal in response to a command sent from an upper-level device such as a receiver or a relay, and transmit the smoke detection signal to the upper-level device. The upper-level device finally judges whether a fire has broken out. The on-off fire detectors transmit a fire detection signal to the upper-level device when detecting that the smoke detection signal has exceeded a fire level.
The conventional photoelectric fire detectors must take measures against noises occurring on a power supply line or noises occurring in the air for fear they may malfunction. Recently, there is an increasing demand for fire detectors unsusceptible to high-frequency noises along with prevalence of portable telephones. Moreover, international standardization organizations including the International Electrotechnical Commission (IEC) tend to stipulate stricter standards concerning noises.
Conventional countermeasures against noises include addition of noise absorption elements to a circuit and devised patterning of a printed-circuit board. In the analog fire detector, a microprocessor (hereinafter MPU) is included for suppressing a variation of a sampled smoke detection signal caused by a noise according to the method of moving averages or the like.
However, the noise-related standards have come to cover high-frequency noises. Moreover, the intensities of the noises have increased. The addition of noise absorption elements or the devised patterning has therefore reached its limitations. Besides, when the MPU is used to reduce a variation of a sampled smoke detection signal according to the method of moving averages, a caused by an instantaneous noise can be reduced. However, when a noise persists for a long period of time, the MPU cannot reduce the noise, at the worst case, a malfunction occurs.
Another countermeasure against noises is such that the printed-circuit etc, board are encapsulated in a shield case in order to block radio waves (refer to U.S. Pat. No. 4,897,634).
In this case, as shown in FIG. 11, the circuitry for a fire detector circuit base 100 is realized in the form of a double-sided printed-circuit board 115 whose lower surface is coated with a copper foil in order to block radio waves. A metallic shield case 120 is placed on the printed-circuit board 115, thus de-influencing the adverse effects of radio waves on circuit elements 116. Moreover, a light-emitting element 108, a light-receiving element 109, and a capacitor 117 mounted on the lower surface of the double-sided printed-circuit board 115 are covered with a light-emitting element holder, a light-receiving element holder 107, and a capacitor holder 114 and thus unsusceptible to radio waves.
However, particularly, the light-receiving element 109 should be protected by the noise. So the light-receiving element 109 was only protected by metal-sealed-cap (not shown), which is in the element holder 107.
In FIG. 11, there are shown a body cover 101, a contact pin 102, an outer cover 103, a smoke inlet 103a, and a smoke-sensing section 104. A wire net 102 prevents invasion of insects into the smoke-sensing section 104.
When a seal member shields for a fire detector heavily, the number of components increases and the number of assembling steps increases. This may lead to an increase in costs.
An object of the present invention is to provide a fire detector that is prevented from malfunctioning because of noises, and a noise de-influence method for the fire detector.